Olfactometers are instruments capable of exactly dosing a portion of the gaseous phase which is present due to the vapor pressure of a sample or in the form of a liquid or a solid, especially an odorant. Generally in all fields, olfactometers are desired which cover a large dynamic range with a very high accuracy.
An apparatus capable of covering a large dynamic range with a very high accuracy is known from S. F. Hallowell et al., Cargo Inspection Technologies, SPIE Vol 2276 (1994), 437-448. This publication discloses an apparatus which has been developed at the Auburn University and was used to study and to train dogs for explosives detection. It is based on a five-fold dilution cascade.
A compressor delivers air at a constant pressure through a filter and is split in two main channels one of which provides odor-free air used for dilution purposes and the other is used as sample carrier. The sample free air is distributed to five mass flow controllers, which in parallel deliver on five different channels air at a constant flow rate. The sample carrier is passed through mass flow controllers with a variable flow rate. After passing the saturation chamber, the carrier gas is split in two new channels, either to reach the sniff-port via passing a valve or to be further diluted by passing again a variable mass flow controller prior to being mixed with sample free air. Five identical dilution cascades are available of which the theoretical dilution limit is 1:1000 per cascade and would enable a dynamic range of 1:10.sup.12. It was shown experimentally that a dilution range of 1:100 per cascade (range &lt;10.sup.8) which only is obtained stepwise.
An important drawback of this system is the obvious path of the sample flow through the mass flow controller units as well as through the valves. The main contamination source of the system is found there. The publication explicitly mentions decontamination procedures. The fact that a mass flow controller is controlled by hot wire detectors, requires calibration data for each gas composition. This makes it impossible to calibrate the system in an exact way. In addition, each inaccuracy in a previous cascade can be amplified by the following dilution step according to error evolution laws.
Moreover, this known olfactometer has only one channel and is, therefore, not capable of performing comparison experiments. Due to the manual operation of the mass flow controllers it is not possible to have a fast and pulse-like stimulus delivery.